1. Field of the Invention
The present invention relates to a radially acting track follower system with an optical scanning provision for an audio or video record replay apparatus, which employs a light source, a rotatable phase grating, a position adjustable reflection mirror provision, and an optical light bundling means for generating three luminous spots disposed substantially on a straight line, which straight line can be adjusted relative to the tangent at the track scanning position.
2. Brief Description of the Background of the Invention Including Prior Art
An optical track follower system for audio and video record players is disclosed in U.S. patent application Ser. No. 06/623,971 filed June 25, 1984 by Hans-Robert Kuhn. An optical scanner and reader for information on optical records is disclosed in U.S. patent application Ser. No. 628,770 filed July 9, 1984 by Arthur Kurz et al, now abandoned.
An optical three beam scanning system, also called a three beam optical pick-up, is known from the German Patent No. DE-PS 2,320,477. The radial error signal required for radial adjustment control is generated by two auxiliary luminous spots disposed in tangential direction relative to the track. These two luminous spots are at about equal distance from the center main luminous spot and the center points of the three luminous spots are approximately disposed on a straight line. They are displaced in a radial direction relative to the center of track such that for an about radial deviation of the track relative to the light spot, opposite effects are generated in the two auxiliary light spots. The reflected light intensities of the two auxiliary light spots are separated from each other and are read separately from the reflected light intensity of the main luminous spot via correspondingly ordinated detection means.
It is further known from the same patent that overall three light spots can be generated with a phase grating, which is provided such that it can be adjusted around a small angle for the purpose of a one time adjustment, as it is known in commercially available equipment, in a plane disposed vertically around a rotation point, which rotation point does not coincide with the center point of the phase grating but is instead disposed outside of the phase grating. A required rotation is performed mechanically via a lever with a set screw accessible from the outside of the apparatus.
The conventionally known optical scanning devices according to the three beam principle are associated with the following disadvantages relative to their track error correcting properties:
(i) They cannot be employed in situations where, based on the construction principle, only one radial control is performed with a tilting lever since the tangential error angle associated herewith results in such a shifting of the position of the auxiliary light spots relative to the track depending on the just scanned track radius that a maintenance of the track can not be achieved.
(ii) In the case of radial linear systems with a coarse and fine track adjustment control, the difficulty results that even in the case of very small deviations in the guiding of the radial adjustment, an unfavorable radial shifting of the the two auxiliary luminous spots relative to the track occurs because of the difference in curvature over the surface of the disc. This results in a defective maintenance of the track and/or can cause a loss of the track being followed.
It is known to employ movable mirrors or movable lens systems for the joined radial adjustment control of all three light beam bundles or, respectively, their luminous spots. Compare in this context the journal Funkschau 19/83, page 49.
It is further known from "Philips Technical Review" Vol. 40 (1982), page 154 that a radial acting control signal can be applied to the radial track follower system for improving the following properties. This radially acting control signal can, for example, be employed at a frequency of 600 Hertz and effect a radial back-and-forth oscillation of the three light spots with an amplitude of about 0.05 micrometers. The radial oscillation motion of the scanning device results in a superposition of the light intensities on the detection elements. By way of a corresponding sum formation, filtering and comparing with the original control signal, one can conclude about a possibly present asymmetry of the optical system and thus errors in the radial error signal can be determined and avoided. Such a provision, however, is associated with the disadvantage that the control signal is superposed on the scanned information signals, that is, it is located within the frequency spectrum of the information content. Therefore it is also not possible to employ a high frequency for the control signal since the energy parts of the information signals initially become larger with frequency. However, it would be very desirable to employ a substantially higher frequency as, for example, 500 or 600 Hertz, since only then a sufficient distance to the frequency spectrum of the radial error signals can be achieved.
It is also known from the initially recited German Patent No. DE-PS 2,320,477 according to FIG. 5 and the corresponding description in columns 5 and 6, that detection elements can be employed in each case with a parted surface. Thus it is possible to determine the intensity distribution of the light impinging on the detection elements and to produce controlled signals based on the formation of the difference. The control signals are then employed for controlling the position of the mirror 59 and 54 in the reference.